The growing interest in vessel maneuverability and propulsive efficiency results in a growing interest in main propulsion apparatus and auxiliary propulsion apparatus equipped in vessels. For example, a vessel like a drillship is equipped with an azimuth thruster for generating thrust in order to implement precise positioning or tow other vessels during high- or low-speed navigation.
There are two variants of azimuth thrusters, based on their use, that is, the open azimuth thrusters (for example, propellers) without a duct, and the ducted azimuth thrusters with a duct having an airfoil section around their propeller.
The aforementioned azimuth thrusters have a gear positioned in the hull capable of rotating in a horizontal direction to generate thrust in all azimuths, that is, omni-directional thrust. It is essential that a drill ship implements accurate DP (Dynamic Positioning) for drilling against environmental loads, for example, wave drift forces due to waves, external forces due to wind, and external forces due to tides.
Also, as the drillship employs an azimuth thruster as an auxiliary propulsion apparatus to go to drilling sites, general operational conditions of the azimuth thruster are also very important. If a great towing force is required in operation, generation of great towing forces depending on towing conditions is also very important.
In particular, vortices take place in the rear center of a propeller when it rotates, and lowers the pressure of fluid flowing into the propeller to generate forces in the direction of hull resistance, thereby reducing the propulsive efficiency of the propeller.
In relation to this, one prior art reference is Korea Laid-open Publication No. 10-2012-0098941, entitled “THRUSTER WITH DUCT ATTACHED AND VESSEL COMPRISING SAME”.
In this prior art, because the sectional shape of the duct is located on the outer surface of the front end of the duct during high-speed navigation, the thruster has a portion expanding with a circular section outward from a standard airfoil to inhibit pressure change, and an open angle of which the direction of leading edge is widened to generate a predefined towing force in low-speed operation.
However, the prior art does not disclose the distance from the parallel portion on the inner side of the duct which is in parallel with the duct axis (e.g., X-axis or the axis of propeller rotation) to each of the nose and the tail. In the prior art, important design variables are not described about what numerical ranges the front portion and the rear portion in the parallel portion belong to on the basis of the position of thruster plane drawn by the rotating end of the propeller blade (plane Y-Z: the plane of propeller rotation). Therefore, the effect of the aforementioned important design variables on total thrust, the torque of a propeller and the exclusive efficiency of an entire thruster is not known. The aforementioned prior art document does not provide enough description to develop a propulsion apparatus that offers even higher propulsive efficiency, while implementing precise maneuverability and highly-efficient towing.
In addition, the prior art mentions just the outward expansion and the open angle of which the leading edge direction is widened, but does not describe any technology for reducing vortices taking place by propellers. In this context, it may be difficult to absorb the rotational component of propeller wake in the bollard condition in which just the propeller rotates at a rated RPM while a vessel or marine structure almost stands still.